**Core Concept**
The external manifestation of an organism's genome refers to the physical expression of its genetic makeup, encompassing all the characteristics and traits that can be observed and measured. This is a fundamental concept in biochemistry, as it highlights the relationship between an organism's genotype (its genetic code) and its phenotype (the physical characteristics that result from the interaction of the genotype with the environment).

**Why the Correct Answer is Right**
The correct answer, **phenotype**, is the external manifestation of an organism's genome because it represents the actual expression of the genetic code. The phenotype is the sum of all the physical and behavioral characteristics that result from the interaction of the genotype with the environment, including factors such as diet, temperature, and exposure to light. For example, the genotype of a tall person may be influenced by multiple genes, but the phenotype is the actual height of the person.

**Why Each Wrong Option is Incorrect**
**Option A:** Genotype refers to the genetic code itself, not the external manifestation of the genome. It is the raw genetic information that determines the phenotype.
**Option C:** An allele is a variant of a gene, but it is not the external manifestation of the genome. Alleles contribute to the overall genotype, which in turn influences the phenotype.
**Option D:** Polymorphism refers to the presence of multiple alleles of a gene in a population, but it is not the external manifestation of the genome. Polymorphism can influence the phenotype, but it is not the phenotype itself.

**Clinical Pearl / High-Yield Fact**
When considering the relationship between genotype and phenotype, remember that the phenotype is the result of the interaction between the genotype and the environment. This is often referred to as the "nature vs. nurture" debate, highlighting the complex interplay between genetic and environmental factors that shape an organism's characteristics.

**✓ Correct Answer: B. Phenotype**